Ground electrodes, in this context, means devices used to connect an electrode line of a power network comprising an HVDC transmission system, via one or more feeder cables, to a conducting medium such as soil or sea water.
HVDC transmission systems usually have DC voltages above 5 kV and a transmitted power above 10 MW.
As compared with alternating current (AC) transmission systems, HVDC transmission systems require only two conductors. At least one of those conductors is implemented as an overhead line or a high voltage cable. For bipolar transmission another conductor of the same kind is used under normal operating conditions, but in monopolar transmission, the ground, that is soil and/or sea water, is used as a return conductor for the transmitted DC-current. However, also in HVDC transmission systems intended for bipolar transmission, ground electrodes are required to transfer unbalance currents and, under operation in monopolar mode, the whole DC current transmitted by the HVDC system.
The ground electrodes can operate as anodes, that is, delivering current to the conducting medium, or as cathodes, that is, receiving current from the medium.
Depending on the location of the HVDC-system, the ground electrodes can be located in soil or in sea water. Ground electrodes located in soil usually have certain advantages as compared to ground electrodes located in sea water. Thus, they are comparatively easy to mount and to have access to in the case of maintenance or repair. At least under normal operating conditions they are also better protected against mechanical damages and will usually not be subject to varying mechanical stresses. The risk that human beings or animals will come into direct contact with the electrode is very small.
This application is concerned with land electrodes, that is, ground electrodes located in soil.
The land electrode, via one or more feeder cables, transfers the DC current from an electrode line of the HVDC system to the soil or vice versa. The soil, in this context, is generally to be regarded as a conducting, however, inhomogeneous medium.
For a general description of ground electrodes in connection with HVDC systems, reference is made, for example, to E. Uhlmann: Power Transmission by Direct Current, Springer Verlag 1975, in particular pages 255-273.
The land electrodes are-apart from the requirements as to current and resistance-also required to be electrically safe, to have high operational reliability and sufficiently long service life, and in addition, to not cause any harmful environmental effects, such as for instance drying up of the soil in the vicinity of the electrode.
The resistance of ground electrodes has to be low, usually well below one ohm. In particular for land electrodes, the step voltage at the surface of the soil in the vicinity of the electrode, which creates a danger to human beings, should be less than a specified level. The step voltage Vs is calculated according to an expression Vs=(5+0.03* .rho..sub.s) volts, where .rho..sub.s is the minimal local specific resistivity (expressed in ohm*m), of the soil located above the electrode.
A conventional land electrode comprises an active part, herein called the electrode body, which is in electric contact with the soil and through which the current is transferred, interconnection cables for internal connection of parts of the electrode body as described below, and additional parts performing mechanical functions, including mechanical protection.
The average current density on the surface of the electrode body is usually not higher than a few A/m.sup.2.
In order to reach a sufficiently low grounding resistance, a land electrode usually comprises a large number of sub-electrodes, each sub-electrode being fed from a separate sub-electrode feeder cable. A sub-electrode comprises a backfill, usually a bed of coke, and an active sub-electrode element, the feeding element, embedded in the backfill. The feeding element is in electric contact with the sub-electrode feeder cable and has an active part of its surface which is in electric contact with the backfill. In cases where the sub-electrode comprises more than one such feeding element, these elements are coupled to each other by feeding element interconnection cables.
The backfill occupies a certain volume around the feeding element and is in its turn embedded in the soil. The active part of the surface of the backfill is that part of its surface which is in electric contact with the soil.
The sub-electrodes are usually arranged in sections, each section being fed from a separate section feeder cable, which is in electric contact with the electrode line. Each section of sub-electrodes may comprise a plurality of sub-sections, each sub-section being fed from a separate section interconnection cable which is in electric contact with the section feeder cable.
The sub-electrodes are arranged along contour lines. A contour line is to be understood as the trace, as seen from above, of a section or a sub-section of sub-electrodes. The contour line of a section or a sub-section of sub-electrodes can typically have the shape of a circular arc, in which case the sections and/or sub-sections of sub-electrodes can be arranged in such a way that the contour lines of the electrode coincide with a circle.
FIG. 1 illustrates schematically an electrical configuration typical for an HVDC transmission system with land electrodes at both ends. An electric alternating current (AC) power network N1 is via a transformer T1 coupled to the AC-side of a thyristor converter SR1 and an AC power network N2 is via a transformer T2 coupled to the AC-side of a thyristor converter SR2. On the DC-sides of the converters, an overhead line LO connects one of their respective poles, and the ground return comprises two electrode lines LE1, LE2, two land electrodes 15 of similar structure, and the soil (not shown) between the electrodes. The land electrode at the converter SR1 comprises a plurality of sub-electrodes 16, each of which is coupled to the electrode line via a feeder cable 29. Each subelectrode comprises a plurality of feeding elements 161,162,163, interconnected by interconnection cables 2', 2", 2'" respectively. The electrode body comprises all the feeding elements 161,162,163 comprised in all the sub-electrodes coupled to the electrode line.
FIG. 2A shows schematically a typical layout, as seen from above, of a land electrode 15 for an HVDC transmission system. The contour line of the electrode is in the form of a circle and the electrode is fed from an electrode line LE1 via three section feeder cables 29a, 29b, 29c. FIG. 2B shows a side view of a part of the electrode, comprising three series connected rod-shaped feeding elements 161,162, 163, with their longitudinal direction in a horizontal direction. Each feeding element is embedded in a layer 170 of backfill in the form of coke, which layer in turn is embedded in a soil layer 28 at some distance below the surface 10 of the soil. All parts of the electrode are similar to the part illustrated in FIG. 2A. FIG. 2C shows a cross section through the electrode along the section IIC--IIC in FIG. 2B. The diameter of the ring can typically be in the order of 1 km. The material of the feeding elements is typically silicon iron or graphite (for electrodes operating as cathodes, also mild steel).
Alternatively, the feeding elements may be arranged with their longitudinal direction in a vertical direction. This is illustrated in FIG. 3A, showing a side view of a part of an electrode of similar ring form as the electrode illustrated in FIG. 2A, with three parallel connected rod-shaped feeding elements 161,162, 163. FIG. 3B shows a cross section through a sub-electrode along the section IIIB--IIIB in FIG. 3A. Each feeding element is embedded in a layer of backfill, arranged as described in connection with FIGS. 2B and 2C.
Typically, the feeding elements are manufactured in the form of rods, which makes them easy to manufacture and to mount. FIGS. 4A-4B illustrate prior art feeding elements, which are designed with an attempt to prolong their service lifetime. FIG. 4A shows a rod-shaped feeding element 161 with two ends 101 and 102 and with a feeder cable 2 coupled to the end 101. At the feeding end 101, the feeding element has an increased diameter and is in addition protected by a sleeve 3, made of a non-conducting material. The active part S of the surface of the feeding element is in this case its total surface less that part of the total surface which is covered by the sleeve. FIG. 4B shows a sub-electrode element similar to the one as shown in FIG. 4A, the only difference being that it is provided with two feeder cables 2a, 2b, one at each end 101,102 respective of the feeding element, and with one sleeve at each end.
Usually, in conventional land electrodes, feeding elements and sub-electrodes of the same kind are used for the totality of the electrode body.
The following disadvantages with known land electrodes have been observed.
Land electrodes for HVDC transmission systems, which transfer comparatively high currents, often cover large areas.
When operating as anodes, the feeding elements tend to dissolve themselves, in particular at their ends, and the connection to the sub-electrode feeder cable, or, as the case may be, to the feeding element interconnection cable, may finally be broken by dissolution of the part of the feeding element comprising the connection.
The coke backfill is deteriorated during the operation of the electrode, so called coke consumption, leading to an increased ground resistance and a decreased lifetime.
Electrodes located in soil will, when operating as anodes, usually cause a decrease of the soil humidity in the vicinity of the surface of the electrode body. Therefore, the current density at the surface of the electrode body is usually restricted to values typically in the range from 0.5 A/m.sup.2 to 1.5 A/m.sup.2, depending on the type of soil, where the lower value is valid for soil layers such as clay. Higher current densities may result in electro-osmotic processes in the soil and, as a consequence, a further decrease of the soil humidity and a corresponding increase of the specific electric resistivity of the soil and possibly irreversible changes of the soil properties in the vicinity of the surface of the electrode body. Another consequence is that the current density on neighboring parts of the electrode body will increase, resulting in that the above mentioned phenomenon can develop along the electrode.
With an extension of the electrode over large areas follows an increased possibility that different parts of the electrode will be located in soil layers with different physical properties. Soil layers, in this context, are to be understood as volumes of soil in which the sub-electrode are embedded. As a consequence, in the vicinity of a soil layer with high conductivity, there is a risk for violation of electric safety regulations. Further, local heating of the soil layer may become high, causing a dry up of the soil, especially in combination with electro-osmotic processes.
To prolong the service lifetime of the feeding elements, various remedies have been proposed, as described above in connection with FIGS. 4A-4B. Thus, it has been proposed to increase the diameter of the feeding element near the feeder cable connection. It has also been proposed to provide the end of the feeding element with a sleeve of a non-conducting material. This measure, however, only moves the zone of dissolution to the edge of the sleeve. It has also been proposed to use two feeder connections, one at each end of the feeding element. These measures have, however, only a limited effect on the service life of the feeding element, achieved at the expense of more complicated and expensive designs.